The term 'strike zone boundary' refers to the area between a batter's KNEES and the midpoint of their torso. The crossword solutions for this term are KNEE (4 letters) and KNEES (5 letters). This term is used in baseball and softball to determine whether a pitch is a strike or a ball. A pitch that passes through the strike zone boundary is considered a strike, while a pitch that does not pass through the strike zone boundary is considered a ball.